Iodine atoms relaxation

Iodine atom abstraction from an alkyl iodide by I 2/ ) is an exothermic process and thus may compete with spin orbit relaxation ... 

Nesbitt and Hynes [57] have pointed out that much of the oscillator strength of the iodine molecule s optical absorption arises from the lowest few vibrational states of the ground electronic state (X). It is necessary for the iodine atoms to recombine and then relax to these low vibrational... 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

Figure H. The calculated dme-dependent total friction (f(r)) and its relative contributions from the binary term ( CB(0) and the density relaxation term RPI> t) for the iodine atom in CH3I. The reduced temperature T (= kaT/f) is 0.363 and the reduced density p for CH3I is 0.918. The time-dependent friction is scaled by t 2, where z,c = [miaj/kBT 2 3.3 ps. The friction is found to be much higher for Iodine than CH3. This figure has been taken from Ref. 133.

Crystal structures for some iodine-substituted compounds were selected from the CSD according to the same criteria applied for chlorine and bromine compounds, except that the number of iodine atoms [criterion (a)] was relaxed to > 1. This search resulted in 18 observations for C(sp3)—I bonds with a mean value of 216.6 pm and o = 2.0 pm. The individual values range from 212.3 to 220.6 pm. 34 observations were listed for C(sp2)—I distances with a mean value of 210.1 pm and a large standard deviation of 6.1 pm. The values range from 205.7 to 223.9 pm. The mean values of this sample are very close to the typical bond distances listed by Allen and Coworkers172. Table 46 lists C(sp3)—I and C(sp2)—I bond distances for some selected crystal structures. 

The band converges to a continuum at 499.0 nm. At this wavelength the energy of the transition is sufficient to bring the molecule to that required to dissociate the molecule into two iodine atoms, one of which is in the excited state, Pi/2 Strictly speaking, the transition from to E[o should be spin-forbidden, but because of the large number of electrons in the I2 molecule this prohibition is relaxed. The transitions from the ground state to... 

Kelley and Rentzepis [297] have recently studied the recombination of iodine atoms in liquid and fluid xenon over times to 150 ps after photolysis. The iodine molecule can be biphotonically dissociated through the state to produce geminate pairs with larger initial separations. Some degree of spin relaxation of excited iodine atoms ( Pi/2) produced by biphotonic excitation may occur and reduce the probability of recombination. There is also evidence that the 11 state of I2 may be collisionally predissociated and that recombination may be more rapid than the rate of vibrational relaxation of the excited 12 state in polyatomic solvents (see also ref. 57). Despite these complications, several workers have attempted to model the time dependence of the recombination (or survival) probability of iodine atom reactions. The simple diffusion equation analysis of recombination probabilities [eqn. 

The results obtained from the irradiation at 235 nm of vibrationally excited dichloromethane suggest that there is higher non-adiabaticity for vibrationally excited molecules.Laser irradiation at 355 nm of methylene chloride and methylene bromide brings about ionization.A simple C-Br bond fission is the key step in the photodissociation of methylene dibromide at 248 nm. Ionization of methylene bromide in the 10-24 eV range has been reported. Irradiation of methylene iodide in water has provided evidence for the formation of iodine atoms and protons.Further work on the photochemical behaviour of methylene iodide has reported that vibrationally relaxed CH2l- T reacts with cyclohexene to afford norcarane. ... 

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